The present invention relates to a drive system for a fastening tool for fastening one or a plurality of workpieces of ductile material, the fastening tool comprising a punch for performing a fastening operation and a clamp axially displaceable relative to said punch and providing a clamping force for clamping said one or said plurality of workpieces during said fastening operation.
Known fastening tools such as tools for setting self-piercing rivets generally use hydraulic drive systems. In such drive systems the force for actuating the punch (the fastening force) is generated by means of a hydraulic cylinder which transmits hydraulic pressure directly or indirectly to the punch, see for example DE 199 24 310.
The clamping force exerted by the clamp upon the workpieces may be generated either by an additional hydraulic cylinder or by the punch via force transmitting means comprising a spring. While these drive systems have been successful in practice, they are not equally well suited for all types of applications. The use of an additional hydraulic cylinder requires substantial structure and complicated control systems. The use of a spring as force transmitting means involves the risk of spring failure resulting in reduced reliability of the fastening tool. Furthermore, the clamping force is fixed by the spring and cannot be varied.
It is an object of the present invention to provide a drive system for a fastening tool which avoids the disadvantages of prior drive systems.
It is a further object of the invention to provide a drive system for a fastening tool which is of simple structure, improved reliability and reduced wear.
It is still another object of the invention to provide a drive system for a fastening tool wherein the clamping force for clamping the workpieces can be varied and individually set.
In accordance with the present invention the force transmitting means between the punch and the clamp comprises an air pressure chamber of variable volume which can be reduced by axial relative movements between the punch and the clamp during the fastening operation in order to compress pressure air therein so as to increase the clamping force. As a result the air pressure chamber acts as a pneumatic spring which generates a predetermined clamping force.
Since the drive system of the present invention does not require a mechanical spring for transmitting forces between the punch and the clamp, the drive system is extremely reliable, exhibits reduced wear and is of increased duration. Furthermore, the invention enables continuously to set the initial pressure within the air pressure chamber to any desired value. As a result the clamping force can be set individually and rapidly and furthermore can be adapted to specific applications. Furthermore, the pressure within the air pressure chamber can be selectively controlled by a variable restriction or pressure control means.
The drive may be a conventional actuator such as a hydraulic cylinder which acts upon the punch either directly or indirectly. However, it is preferred that the drive comprises an electric motor and a spindle mechanism driven by the electric motor.